Mosquito season in Franklin County, PA: when it starts and ends

Mosquito months in Franklin County

In a normal year mosquito season in Franklin County, PA starts around April 8 and ends around October 28, about 29 weeks.

1. Jan
2. Feb
3. Mar
4. Apr
5. May
6. Jun
7. Jul
8. Aug
9. Sep
10. Oct
11. Nov
12. Dec

What to do in Franklin County, and when

• Start tipping out standing water weekly by March 25.
• Book any yard treatment for early April, ahead of the July peak.
• Repellent can go away after October 28.
• 12 West Nile cases since 2016: you should wear EPA-registered repellent at dusk June to August, and check PA spray notices.

Franklin County against the rest of PA

Franklin County has the 7th longest season of 67 PA counties, at 203 days against 222 in Philadelphia County and 154 in Clinton County. Its mean temperature is 53.1°F.

• Lancaster County: April 9 to October 29 (204 days, 44 WNV)
• Bucks County: April 10 to October 31 (204 days, 28 WNV)
• Adams County: April 11 to October 27 (200 days, 2 WNV)
• Allegheny County: April 12 to October 27 (198 days, 23 WNV)

West Nile in Franklin County

From 1999 to 2025, CDC ArboNET counted 29 human West Nile cases in Franklin County (23 neuroinvasive) across 12 years. The worst was 2018 with 8; the latest was 2022.

Background: reading the West Nile counts; how the dates are worked out.